Several United States patents have issued to the applicant for drying coal in a fluidized bed reactor. These include U.S. Pat. No. 5,830,246 (“Process for processing coal”), U.S. Pat. No. 5,830,247(“Process for processing coal”), U.S. Pat. No. 5,858,035 (“Process for processing coal”), U.S. Pat. No. 5,904,741 (“Process for processing coal”), and U.S. Pat. No. 6,162,265 (“Process for processing coal”). The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification.
With increasing energy demands, and increasing energy production costs, there is a need for efficient production methods for upgrading low rank or “wet” coal products to consumable energy products. Many researchers have devoted significant resources to developing these processes and technologies.
The coal industry has faced excessive transportation costs for these moisture-laden low-rank coal products. However, while drying coal to a low moisture content prior to shipment offers significant advantages in terms of reduced transportation costs, it renders the coal subject to spontaneous combustion during shipment and storage. Significant inflagration and explosion hazards are created, exposing workers and emergency responders to dangerous conditions.
The problem of spontaneous combustion of coal has been well known for more than half a century. Sub-bituminous, bituminous, lignite, brown coal and coal char can spontaneously combust by chemical reactions between the coal, moisture and oxygen present in the air. This reaction can occur when water combining with other components in the coal generate a sufficient amount of heat to raise the temperature of the coal to the ignition point. Additionally, noncarbonaceous or unsaturated carbon compound materials present in the coal may oxidize upon exposure to air, which in turn generates a sufficient amount of heat for the coal to reach ignition temperature.
U.S. Pat. No. 4,170,456 (Inhibiting spontaneous combustion of coal char) explains that, “Spontaneous combustion occurs when the rate of heat generation from oxidation exceeds the rate of heat dissipation. Previous workers have found that the reason spontaneous combustion does not occur more often than it does is that the oxidation rate of coal char decreases with the increasing time of or extent of oxidation. Therefore, when coal char is exposed to oxygen, a race begins between the effects of high temperature coefficient of oxidation rate and the decreasing rate of oxidation as oxygen is consumed by the coal char. Depending on the winner, spontaneous combustion occurs or doesn't occur.” The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification.
Commonly used drying processes utilize a hot combustion gas to drive moisture from the coal in a bed of coal, a fluidized bed, a kiln or a rotary device. Conventional drying methods often center around pyrolysis and result in a coal product which is active and subject to self-heating by the processes described above.
U.S. Pat. No. 6,146,432 (Pressure gradient passivation of carbonaceous material normally susceptible to spontaneous combustion) explains “Low-rank coals, such as sub-bituminous coal or lignite may contain more than about 10% moisture and typically 15-50 weight percent moisture. Some low-rank coals may contain as much as 60 weight percent moisture. Such wet low-rank coals cannot be shipped economically over great distances due to the cost of transporting a significant fraction of unusable material in the form of water. Further, these low-rank coals cannot be burned efficiently due to the energy required to vaporize the water. Due to the lowered heating value and high cost of shipping unusable material, it is advantageous to remove all or part of the water from the low-rank coals prior to shipment and/or storage. However, drying such fuels usually leads to activation of the low-rank coals or chars. The reactive coals or chars may be hazardous due to the potential for damage to property or life due to the reaction of the coal or char with atmospheric oxygen and moisture and consequential heating of the coal, which makes it subject to spontaneous ignition during either shipment or storage. Indicators of the propensity of coals or chars to spontaneously combust include the uptake of oxygen as measured in terms of torr of oxygen per gram of material. Methods for testing this indicator are listed in U.S. Bureau of Mines “Report of Investigation 9330” by Miron, Smith, and Lazzara. The terms “oxygen uptake” and “oxygen demand” refer to the test methods of the “Report of Investigation 9330” or related test methods when used in this document. In the past, wet low-rank coals such as those from the western United States have been dried by methods such as, but not limited to, thermal drying using process heat, waste heat, microwaves, pressurized water, steam, hot oil, molten metals, and other supplies of high temperatures. The heated coals release the free moisture trapped in the pores, water molecules associated with hydrated molecules or associated in other ways with the coal, producing dried coals or chars. Other methods of drying may include mechanical drying (such as centrifugal separation), the use of dry gases, or the use of desiccants or absorbents. Once dried, coals or chars can become more active and are known to spontaneously combust.” The entire disclosure of said United States patent is hereby incorporated by reference into this specification.
Many researches have devoted significant resources to address this problem, some of which will be briefly described. None of the approaches, and in particular, those utilizing an oxygenated environment, have realized commercial success.
To reduce the potential for the spontaneous combustion of coal, approaches have focused on filming or coating the surface of the coal with deactivating fluids to seal it using oils, polymers, tars, waxes or other hydrocarbon materials. Reference is made, e.g., to U.S. Pat. No. 1,960,917 (Process for treating coal), U.S. Pat. No. 2,197,792 (Coal spraying chute), U.S. Pat. No. 2,204,781 (Art of protecting coal and like), U.S. Pat. No. 2,610,115 (Method for dehydrating lignite) and U.S. Pat. No. 2,811,427 (Lignite fuel). U.S. Pat. No. 3,961,914 (Process for treating coal to make it resistant to spontaneous combustion) disclosed a silicon dioxide film on the coal surface. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. Without wishing to be bound by any particular theory, applicants believe that favorable altering of the surface components reduces the reactivity and oxidation.
Other methods have used application of oxidizing agents or treatment with high temperature under pressure (U.S. Pat. No. 6,146,432 at column 2, lines 35-60). The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. Yet other processes use controlled drying in a manner that particle surface pores are self-sealed by hydrocarbon material evolved from the particles.
Other approaches include the prolonged exposure of the coal to air, the use of oxidizing agents sprayed on coal, and treating the coal with high-temperature water under pressure. The coatings perform their work by covering the pores and limiting the access of active components of the air to active sites on the dried coal. U.S. Pat. No. 3,723,079 (Stabilization of coal) explains: “For example, coal piles are often arranged in a particular manner to obtain safe storage; e.g., thin layers which are compacted with sloping sides at a maximum angle of 14°, smooth final surfaces, and top surface continually smoothed as coal is removed from the top only. Other approaches to prevent spontaneous combustion during storage involves chemical treatment of the coal, e.g., coating the coal with petroleum products and their emulsions, spraying with calcium bicarbonate or aqueous hydroquinone or amines. Such treatments, however, are either not completely effective or are excessively expensive for a low prices commodity such as coal.” The entire disclosure of such United States patent is hereby incorporated by reference into this specification.
U.S. Pat. No. 3,985,516 (Coal drying and passivation process) and U.S. Pat. No. 3,985,517 (Coal passivation process) disclose mixing of coal in a fluidized bed with at least 0.5 weight percent of hydrocarbon material during the heating process. These coatings are effective in preventing reabsorption of moisture, however, such coatings are expensive due to the cost of the added hydrocarbon materials. The entire disclosure of said United States patent is hereby incorporated by reference into this specification.
U.S. Pat. No. 1,632,829 (Method of drying coal and the like) describes a process for drying wet coal by steam heating it. In the method described, steam disposed above the coal is maintained at high partial pressure to prevent escape of the moisture while the coal temperature elevates. Thereafter, the steam pressure is reduced, permitting the escape of moisture and rapid drying of the coal. The entire disclosure of said United States patent is hereby incorporated by reference into this specification.
U.S. Pat. No. 4,052,169 (Treatment of solid fuels) describes a process for upgrading lignitic coal by heating it in an autoclave at about 750° F. and pressures in excess of 1000 psig to effect thermal restructuring. Thereafter the coal is cooled and condensable organic material is deposited on the lignite, stabilizing it and render it non-hygroscopic and more resistant to weathering and oxidation during shipment and storage. It is believed that the use of high temperature water drives off carboxylic acid groups and rendering those sites inactive to future activity with the active components of the fluid. The entire disclosure of said United States patent is hereby incorporated by reference into this specification.
U.S. Pat. No. 4,214,875 (Coated coal piles) disclosed a coating composition to be applied to a pile of coal exposed to the weather in order to exclude rain and air by forming a continuous covering over the pile. The composition was normally thixotropic and included wax, tar or pitch or a polymer which provided a covering from one-quarter inch to one inch thick. It was necessary to break the covering in order to transfer or utilize the coal. The entire disclosure of said United States patent is hereby incorporated by reference into this specification.
Berkowitz in Canadian patent 959783, described a method of treating low-rank coals which included heating the coal to a temperature (about 350° C.) by immersion in a liquid medium, causing pyrolytic material to diffuse from the interior to the surface of the coal particles and to plug to pores to prevent moisture reabsorption. The entire disclosure of said Canadian patent is hereby incorporated by reference into this specification.
Wong disclosed in U.S. Pat. No. 4,461,624 (Beneficiation of low-rank coals by immersion in residuum) a process of immersing coal in residuum having a softening point of at least 80° C., at a temperature from about 240° C. to the decomposition temperature to boil off the moisture content and coat the coal particles within the immersion medium. This process has the disadvantages of providing a thick coating of treatment material on the coal particles which must be drained off of the particles.” The entire disclosure of said United States patent is hereby incorporated by reference into this specification.
U.S. Pat. No. 6,146,432 (Pressure Gradient Passivation of Carbonaceous Material Normally Susceptible to Spontaneous Combustion) describes a process for the passivation of a carbonaceous material by exposure to an oxygenated gas over a pressure gradient. The entire disclosure of said United States patent is hereby incorporated by reference into this specification.
U.S. Pat. No. 4,170,456 (Inhibiting spontaneous combustion of coal char) discloses a treatment with air and carbon dioxide at temperatures from 50° F. to 300° F. to deactivate the surface of the coal char. The entire disclosure of said United States patent is hereby incorporated by reference into this specification.
U.S. Pat. No. 4,192,650 (Process for drying and stabilizing coal) discloses a treatment that rehydrates the coal to a moisture level of 2-10 weight percent. The entire disclosure of said United States patent is hereby incorporated by reference into this specification.
U.S. Pat. No. 5,527,365 (Irreversible drying of carbonaceous fuels) discloses a method for drying coal in a mildly reducing lower alkane gaseous atmosphere at a temperature of 150° to 300° C., with or without agglomeration with small amounts of oil. The entire disclosure of said United States patent is hereby incorporated by reference into this specification.
U.S. Pat. No. 4,213,752 (Coal drying process) discloses a single-step process using in-situ generated thermal energy and causing partial combustion of the coal at atmospheric pressure in the presence of gas such as atmospheric air. The entire disclosure of said United States patent is hereby incorporated by reference into this specification.
U.S. Pat. No. 4,043,763 (stabilization of dried coal) discloses a process of combining completely or partially dried coal with as-mined coal in a weight ratio of 1:2 to 10:1. The entire disclosure of said United States patent is hereby incorporated by reference into this specification.
U.S. Pat. No. 3,723,079 (Stabilization of coal) discloses a process of treating dried coal with 0.5-8% oxygen by weight at a temperature of 175° C. to 225° C. and rehydrating the coal with water of from 1.5%-6% by weight of oxygen treated coal. The entire disclosure of said United States patent is hereby incorporated by reference into this specification.
U.S. Pat. No. 4,249,909 (Drying and passivating wet coals and lignite) discloses a staged process of heating under low partial pressure of moisture to 8-12% moisture content then heated to a lower differential vapor pressure to remove additional moisture. The entire disclosure of said United States patent is hereby incorporated by reference into this specification.
U.S. Pat. No. 3,896,557 (Process for drying and stabilizing coal) discloses a process of heating the coal in a fluidized combustion gas streat containing 7-9% by volume of oxygen to reduce moisture content to 8-12% by volume. The entire disclosure of said United States patent is hereby incorporated by reference into this specification.
The novel process described in this patent application provides a process for reducing the predisposition of coal to self-heat in the presence of oxygen. This novel, cost-effective and efficient process for irreversible drying and passivation of coal combines the advantages of the coating technology with the exposure of the coal to an oxygenated environment.
While the process taught in U.S. Pat. No. 6,146,432 requires a gradient of pressures, the novel process herein described can be done at atmospheric pressure and moderate temperatures in the range of 450-650 degrees Fahrenheit. U.S. Pat. No. 5,527,365 (Irreversible drying of carbonaceous fuels) teaches that processes involving high temperatures and pressures are economically undesirable, require substantial energy and capital investments and present inherent risks and dangers. The production costs are increased by specialized expensive equipment, apparatuses and facilities. The entire disclosure of said United States patent is hereby incorporated by reference into this specification.
U.S. Pat. No. 4,213,752 (Coal drying process) discloses advantages that are shared by the present invention through a new and novel process, “The process of the invention has the additional benefit that it is less costly because it uses the in-situ generated thermal energy for drying the added wet coal. This results from the fact that no capital investment is needed. Also, the system of the invention allows greater flexibility in the degree to which coal drying is made to occur because the coal stability is not critically sensitive to a particular moisture level and thus the product coal is very highly stable totally dry or with various moisture levels. Still further there is no need in the process of the invention for a rehydrating step which some prior art processes require to obtain a stabilized coal.” The entire disclosure of said United States patent is hereby incorporated by reference into this specification.